The present invention provides a process for treating a chromium alloy or a chromium coating to (a) increase the hardness of the coating or alloy, and/or (b) to increase resistance to friction and wear. The invention also relates to coatings formed using the process, which have superior hardness and friction and wear properties.
Replating of chromium coatings is costly and the effluent from typical chromium coating processes is toxic to the environment. In fact, environmental legislation mandates substantial reductions in chromium plating. As a result, processes are needed to prolong the life of chromium coatings and to reduce or eliminate the need for stripping, re-plating, and re-machining chromium coated tools.
One relatively low temperature process that is currently used to prolong the life of chromium coatings is nitrogen ion implantation, which produces a fine dispersion of relatively hard and stable chromium nitrides, CrN and Cr2N. Unfortunately, chromium nitrides are not as hard as many other nitrides. For example, CrN has a bulk hardness of only about 11 GPa, or half the hardness of TiN. Also, high doses of nitrogen ions are required to produce the chromium nitrides. Finally, nitrides do not generally have a low coefficient of friction.
Thermally-sprayed chromium oxide has been used to coat bearings, but the coating is brittle, is not smooth enough for many applications (such as bearings), and presents the problem of decohesion. In addition, thermally-sprayed coatings are generally porous and therefore do not provide good corrosion protection for substrates such as steel. A need exists for chromium alloy surfaces and non-porous chromium coatings with increased bulk hardness and low friction and resistance to wear.
In one aspect, the invention provides a method of forming a lubricious outer surface comprising chromium, said method comprising:
providing a substrate comprising a surface comprising chromium, said surface having an initial coefficient of friction in an unlubricated condition against a steel counterface; and
treating said substrate with an additive comprising an element X under conditions effective to produce a mixture comprising chromium-X molecules and molecules of said substrate adjacent to said lubricious outer surface, wherein said lubricious outer surface comprises a sufficient quantity of said chromium-X molecules to produce a final coefficient of friction in an unlubricated condition against a steel counterface that is less than said initial coefficient of friction of said surface,
said additive being selected from the group consisting of
substituted or unsubstituted metal carbonyls comprising a metal selected from the group consisting of tungsten, molybdenum, chromium, iron, and nickel, wherein said substituted carbonyls comprise an oxygen of the carbonyl substituted by an element selected from the group consisting of X; and
compounds having the general formula
HnCmXo 
xe2x80x83wherein
n is from about 0 to about 6;
m is from about 1 to about 2;
o is from about 1 to about 2; and,
X is selected from the group consisting of fluorine, oxygen, sulfur, and chlorine.
In another aspect, the invention provides a variety of substrates comprising a gradient from an inside to an outside surface consisting essentially of:
molecules of said substrate/a mixture comprising said substrate molecules and said chromium-X molecules/a surface comprising a sufficient quantity of said chromium-X molecules to produce a final coefficient of friction in an unlubricated condition against a steel counterface that is less than a virgin coefficient of friction of said surface in the absence of said gradient;
wherein X is selected from the group consisting of fluorine, oxygen, sulfur, and chlorine.